Secondary recrystallization of silicon iron with nitrogen

ABSTRACT

THIS INVENTION DISCLOSES THE PRODUCTION OF SILICON-IRON SHEET MATERIAL USEFUL AS A MAGNETIC MATERIAL IN ELECTRICAL APPLICATIONS AND WHICH HAS THE CUBE-ON-EDGE OR (110) (001) CRYSTALLOGRAPHIC GRAIN ORIENTATION. CONVENTIONALLY, THIS STRUCTURE IS PRODUCED BY A PHENOMENON KNOWN AS SECONDARY RECRYSTALLIZATION WHICH HAS DEPENDED UPON A DISPERSION OF PARTICLES OF SULFUR COMPOUNDS WHICH ARE DIFFICULT TO REMOVE. IT HAS BEEN FOUND THAT NITROGEN COMPOUNDS MAY BE SUBSTISTUTED FOR THE SULFUR COMPOUNDS AND THE DESIRED STRUCTURE PRODUCED. THE NITROGEN IS MUCH EASIER TO REMOVE.

limited States Patent 3,575,739 SECONDARY RECRYSTALLIZATION 0F SILICON IRON WITH NITROGEN Howard C. Fiedler, Schenectady, N.Y., assignor to General Electric Company No Drawing. Filed Nov. 1, 1968, Ser. No. 772,857 Int. Cl. H01f N16 US. Cl. 148-111 7 Claims ABSTRACT OF THE DISCLOSURE This invention discloses the production of silicon-iron sheet material useful as a magnetic material in electrical applications and which has the cube-on-edge or (110) [001] crystallographic grain orientation. Conventionally, this structure is produced by a phenomenon known as secondary recrystallization which has depended upon a dispersion of particles of sulfur compounds which are diflicult to remove. It has been found that nitrogen compounds may be substituted for the sulfur compounds and the desired structure produced. The nitrogen is much easier to remove.

SECONDARY RECRYSTALLIZATION OF SILICON IRON WITH NITROGEN The sheet material to which this invention is related is usually referred to in the art as electrical sheet steel and is composed primarily of iron alloyed from about 1.5 to 4.5 weight percent silicon. These materials also contain relatively minor amounts of impurities such as sulfur, manganese, phosphorus and carbon. Since the alloys are principally iron in composition, they crystallize in the body-centered cubic crystallographic system at room temperatures. As is well known, this refers to symmetrical distribution or arrangement which the atoms forming the individual crystals or grains assume in such materials. The smallest prism possessing the full symmetry of the crystal is termed the unit cell and is cubic in form. This unit cube is composed of nine atoms, eight arranged at the corners of the unit cube with the remaining atom positioned at the geometric center of the cube.

Each unit cell in a given grain or crystal in these materials is substantially identical in shape and orientation with every other unit cell comprising the grain.

It is well known that silicon-iron alloys may be fabricated by various rolling and heat treating procedures to netic properties superior to those of a sheet in which the constituent grains are randomly oriented. One of the important oriented magnetic sheet materials has been that possessing the cube-on-edge or (110) [001] crystallographic orientation. In such material, the component grains are oriented so that four of the cube edges of the un1t cells are substantially parallel to the plane of the sheet and to the direction in which it was rolled and a (110) crystallographic plane is substantially parallel to the plane of the sheet. It will thus be seen that the easiest direction of magnetization is in the plane of the sheet parallel to the rolling direction and the next easiest dlirection of magnetization is in the transverse to rolling direction. Conventionally, these polycrystalline grain oriented silicon-iron alloys have been manufactured by castlng ingots of appropriate compositions, hot rolling to an intermediate thickness, followed by cold rolling to the final gauge with a number of heat treating steps. The development of the desired texture or grain orientation has been accomplished by a two step final anneal in which the first step involves a decarburization treatment during which time the cold rolled sheet or strip undergoes primary recrystallization. During this anneal, the presence of a dispersion of fine particles of manganese sulfide prevents undesirable grain growth and maintains small primary grains. During the second step, the manganese sulfide particles are decomposed and the sulfur removed and secondary recrystallization occurs producing the desired preferred orientation. The decomposition of the sulfides and the removal of the sulfur is a relatively slow process and requires heat treatment for several hours to effect complete secondary recrystallization and sulfur removal. It would be desirable to decrease the length of time necessary to achieve secondary recrystallization whereby the cost of the annealing step would be reduced and so that a continuous strip or strand anneal could be used.

It is therefore a principal object of this invention to produce an iron-base silicon alloy which can be processed in shorter periods of time than conventional iron-silicon alloys to produce sheet or strip material having a preferred cube-on-edge grain orientation.

Other objects and advantages of the present invention will be in part apparent to those skilled in the art and in part explained in the following specification.

In order to more clearly and completely disclose the invention, a direct comparison will be made with the conventional production of such materials.

Conventionally, ingots are cast from silicon-iron alloys containing about 0.025 percent by weight carbon, about 0.02 percent sulfur and about 0.05 to 0.06 percent manganese with minor amounts of other impurities which do not affect the processing. These ingots are hot rolled from about 2400 F. to bands about 80 mils in thickness, finishing at about 1600 F. The hot rolled band is then coiled and heat treated in a non-oxidizing atmosphere at about 1650 F. The heat treated band is then cold rolled to about 25 mils in thickness, coiled and annealed at about 1700 F. The annealed material is then cold rolled to a final thickness of about 12 mils and heat treated at about 1400 to 1500 F. in wet hydrogen to reduce the carbon content to about 0.003 percent and to cause primary recrystallization. This anneal may be either a batch anneal or a continuous strip anneal. During this step, the presence of the dispersion of fine particles of manganese sulfide restrains the undesirable growth of the primary grains. The maintenance of the fine grain structure has been found essential in the production of highly oriented material. The decarburized, recrystallized materials are coiled and then box annealed in dry hydrogen at about 2100 to 2200" F. for several hours. During this anneal, the

, sulfur is removed and secondary recrystallization takes same manner as the conventional practice set forth previously. The hot rolled band is then cold rolled to about 25 mils thickness without the conventional heat treatment preceding the cold rolling. The heat treatment of the hot rolled band is not desirable since it tends to cause the silicon nitride particles to locate at the grain boundaries instead of the desired uniform dispersion. The 25 mil thick cold rolled strip is heat treated in the conventional manner and cold rolled to final thickness. The cold rolled material is decarburized in wet hydrogen as previously described and then heat treated in a nitrogen or an inert or noble gas atmosphere at about 1750 to 1850 F. to develop the cube-on-edge texture by secondary recrystallization. The strip is then heat treated in dry hydrogen at temperatures between 1800 to 2000 F. to drive off the nitrogen. The rate of removal of the nitrogen is high enough that it may be accomplished in a continuous strand anneal instead of the conventional box anneal necessary for the removal of sulfur.

As specific examples of the invention, a number of one pound ingots were prepared from vacuum melted highpurity electrolytic iron and high-purity silicon. After the charge was molten, nitrogen was admitted to the desired pressure and the heat was immediately poured. All the heats contained 3.1 percent by weight silicon and not more than 0.003 percent sulfur or 0.005 percent aluminum. The nitrogen and oxygen contents of the resulting ingots were determined by vacuum fusion analysis and are listed in the following table.

It was found that these values do not represent nitrogen contents obtainable under equilibrium conditions. For example, the ingot poured immediately after one atmosphere of nitrogen was admitted to the chamber contained 0.0093 percent nitrogen, whereas an ingot poured after three minutes after the nitrogen was admitted contained 0.021 percent nitrogen and another poured after a six minute delay contained 0.029 percent nitrogen.

The ingots were heated to about 2400 F. in hydrogen and rapidly hot rolled to 0.080 inch in three passes. The hot-rolled pieces were processed both as-hot-rolled and after heating for 3 minutes at about 1650 F. in hydrogen. After cold rolling to 0.026 inch, the strips were heated for 2 minutes at about 1650 F. in hydrogen, then cold rolled to the final gauge of 0.012 inch. The loss of nitrogen in going from the ingot to the finished cold rolled strip was no more than percent. The final heat treatment, which was for the purpose of developing grain orientation, consisted of heating in nitrogen for 2 hours at elevated temperatures. It was found by examination of the macrostructures of strips which had not been heat treated between the hot rolling and cold rolling steps and which had a final texture developing heat treatment of 2 hours at about 1760 F. that the 0.0003 percent nitrogen heat showed normal grain growth with little or no secondary recrystallization, the 0.0093 percent nitrogen heat had a mixed structure of normal grain growth and secondary recrystallization, the 0.0184 percent nitrogen heat complete secondary recrystallization and the 0.0295 percent nitrogen heat incomplete secondary recrystallization. {The latter material did however undergo complete secondary recrystallization after 16 hours at 1760 F.

It was found that even a brief heat treatment after hot rolling greatly diminished the ability of the strip to undergo secondary recrystallization at final gauge. For example, the following table lists estimated values for the percent of cube-on-edge texture after a final anneal in nitrogen for 2 hours at 1760 F. and at 1832 F.

1 Secondary recrystallization incomplete.

From microscopic examination it was observed that heat treatment after hot rolling but before cold rolling produces relatively massive particles of Si N at the grain boundaries which are elfectively unavailable to subsequently form the small, uniformly dispersed particles which are a prerequisite for secondary recrystallization.

From all the foregoing, it will be readily apparent to those skilled in the art that electrical grade cube-on-edge grain oriented polycrystalline silicon-iron sheet or strip material may be produced more economically by substituting Si N particles for the conventional manganese sulfide particles. The invention is applicable to the processing of ingots containing about 1.5 to about 5.0 and preferably from about 2.5 to about 4.0 percent by weight silicon, from about 0.01 to about 0.03 and preferably from about 0.01 to about 0.02 percent by weight nitrogen, not more than about 0.005 percent by weight sulfur, not more than about 0.03 percent by weight carbon, with the balance substantially all iron containing a small amount of manganese and the usual impurities customarily found in such alloys. The exact amount of manganese is considered to be relatively immaterial as long as it does not exceed levels usually encountered in such materials made in accordance with good melting practices for these alloys.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A method for making a polycrystalline sheet-like body having a majority of the constituent grains thereof oriented in the [001] direction comprising the steps of providing an ingot consisting essentially of from about 1.5 to about 5.0 weight percent silicon, from about 0.01 to about 0.03 weight percent nitrogen, less than about 0.005 weight percent sulfur, less than about 0.05 weight percent carbon and the balance substantially all iron containing a small amount of manganese and the usual impurities customarily found in such alloys, the nitrogen being in the form of Si N particles, hot rolling said ingot to an intermediate thickness band, initiating cold rolling of said hand without heat treatment and while the Si N particles are small and uniformly dispersed in the band, cold rolling said band to final gauge material and heat treating said final gauge material to reduce the carbon content to less than 0.005 weight percent, to develop a (110) [001] preferred orientation by secondary recrystallization, and to substantially completely remove the nitrogen.

2. The method of claim 1 wherein the silicon content of said ingot is between about 2.5 and about 4.0 weight percent and said nitrogen content is between about 0.01 and about 0.02 weight percent.

3. The method of claim 1 wherein said heat treatment to reduce the carbon content is accomplished in wet hydrogen.

4. The method of claim 1 wherein said heat treatment to develop said preferred orientation is accomplished in an inert atmosphere.

5'. The method of claim 4 wherein said inert atmosphere is a gas selected from the group consisting of nitrogen and the noble gases.

6. The method of claim 1 wherein said heat treatment to remove nitrogen is accomplished in an atmosphere of dry hydrogen.

tions.

References Cited UNITED STATES PATENTS Fast 1481 11 Wiener 148-113X Fiedler 1481 11 Albert et al 148-1 11 10 3,184,346 5/1965 Fiedler 148111X 3,214,303 10/1965 Fiedler 148'111 3,287,183 11/1966 Taguchi et a1. 148-111 L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant Examiner US. Cl. X.R. 

